This invention relates to a method of miscible flooding for enhanced oil recovery wherein the flood is conducted at flow velocities greater than the critical velocity of flow through the reservoir in order to obtain maximum hydrocarbon recovery within a reasonable period of time. This enhanced oil recovery method is especially applicable to reservoirs having a relatively small dip angle of less than about ten degrees.
The most successful miscible enhanced oil recovery floods are conducted at frontal advance velocities equal to or less than critical velocity. These are termed gravity-stable floods. Miscible floods conducted with carbon dioxide, nitrogen or light hydrocarbon components are generally operated below critical velocity since it is well known in the art that much better oil recovery can be obtained. Above critical velocity, density and viscosity differences between the displacing fluid and the displaced fluid are such that the displacing fluid will tend to finger through the fluid to be displaced in the reservoir in an erratic manner and result in a flood without substantial vertical and horizontal conformance.
Consequently, the prior art recommends miscible flooding at or below critical velocity in order to maximize oil recovery. This is recognized in U.S. Pat. Nos. 3,878,892; 4,136,738; 4,257,650; 4,299,286; 4,418,753 and 4,434,852.
A different twist on flooding at frontal velocities below critical velocity is disclosed in U.S. Pat. No. 4,136,738. In this method, a light hydrocarbon slug of C.sub.2 -C.sub.6 aliphatic hydrocarbons is injected at a rate in excess of the critical velocity to ensure mixing of the light hydrocarbons with the reservoir oil in the area surrounding the injection well. As a result of this mixing, the reservoir oil is altered so as to form a conditionally miscible transition zone which will be miscible with subsequently injected carbon dioxide. Thereafter, carbon dioxide is injected at a relatively low rate which will produce frontal advance velocities less than critical velocity.
Some miscible floods have been conducted at frontal velocities greater than the relevant critical velocity. One example is a miscible carbon dioxide flood conducted by Shell Oil Corp. in the Little Creek Field in Mississippi, wherein the carbon dioxide velocity within the reservoir has exceeded critical velocity for the entire miscible flood. Canadian Patent No. 791,463 discloses a predominantly methane flood with added C.sub.2 -C.sub.6 hydrocarbons conducted under miscible conditions at a velocity greater than critical velocity for the entire flood.